For some years we have been using the unique tool of inbred guinea pigs available at the NIH for investigation of particular areas of the immune responses concerned with mechanisms of hypersensitivity and protection against infectious diseases. Advantages lie in the ability to perform viable adoptive transfers of lymphoid cells and in the capacity to study genetic factors in the hypersensitivity phenomena. For these reasons, we have concentrated recently on the pathogenesis of an age-dependent, strain-dependent chronic autoimmune encepholomyelitis which is strongly reminiscent of multiple sclerosis in man and on the prevention and treatment of this condition using various tolerance-inducing techniques, in particular immune deviation approaches.